Two families of antimicrobial polypeptides have been isolated from horseshoe crabs (see, for example, Shigenaga, 1990, J. Biol. Chem. 265:21350-21354; Kawano et al., 1990, J. Biol. Chem. 265:15365-15367; Muta et al., 1990, J. Biochem. 108:261-266; Japanese Laid-Open Patent Publication No. 167230/1990; Japanese Laid-Open Patent Publication No. 152987/1990; Japanese Laid-Open Patent Publication No. 53799/1990; Published Searched Application 500194/1990; Miyata et al., 1989, J. Biochem. 106:663-668; Akaji et al., 1989, Chem. Pharm. Bull. 37:2661-2664; Taisha (Metabolism) 26:301-311 (1989); Shieh et al., 1989, FEBS Lett. 252:121-124; and Nakamura et al., 1988, J. Biol. Chem. 263:16709-16713). One family, the tachyplesin family has been isolated from the Japanese horseshoe crab Tachypleus. Three tachyplesins, I, II, and III have been identified; their amino acid sequences are shown in FIG. 1. Additionally, a tachyplesin peptide derivative with a carboxyl-terminal extension of glycyl lysine has been found in a Southeast Asian horseshoe crab species, carcinoscorpius rotundicauda (Muta et al., 1990, J. Biochem. 108:261-266). A second family, the polyphemusin family has been isolated from the hemocytes of the American horseshoe crab, Limulus Polyphemus. Two polyphemusins, I and II have been identified; their amino acid sequences are also shown in FIG. 1. The polypeptides in both families consist of 17 or 18 amino acid residues and have four conserved regions in common and two disulfide bridges (see FIG. 1). Both tachyplesins and polyphemusins have been found to inhibit the growth of both Gram-negative and -positive bacteria at low concentrations as well as fungi, such as Candida albicans and form complexes with bacterial lipopolysaccharides (Shigenaga et al., 1990, J. Biol. Chem. 265:21350-21354 and Muta et al., 1990, J. Biochem. 108:261-266). Also, the polypeptides in a tachyplesin family have been found to exhibit some inhibition activity for virus, such as Influenza virus, vesicular stomatitis virus (Murakami et al., 1991, Chemotherapy 37:327-334) or human immunodeficiency virus (Morimoto, et al., 1991, Chemotherapy 37:206-211).